Neck tension - looking at it the wrong way?

[dryhumor]

Apparently the question has been answered to some and I missed it...the bullet moves before the case opens. Or so it would seem. Maybe it's opened for long, heavy bullets but not yet for the short and stubbies, or with a long freebore.

IDK, so I'm asking and would like a better understanding too. If it's open, neck tension, by whatever term, should not affect tune. If not, then yes, especially if jumping. I would conclude from that single statement, jamming is a good thing. Again, I'm asking. I think we must first conclude if the neck is open or not..or somewhere in between, before we can go forward with this subject but that's just my 2 cents and worth less,, probably.

The pressure in a vessel (case) should be common to each surface within the void (static pressure) in the case. Expansion should exert a pressure force that increases at ignition. As the pressure rises it should reach and surpass the pressure (tension) of the case neck, pushing the bullet forward. As the explosion within the case increases more volume is required to allow gas expansion. The case neck then expands. The gases follow the bullet down the bore.

That’s how I visualize it.

ETA After reading mikecr reply I can visualize with that also. The brass being softer and less durable and able to move first.

But what about the shoulders? Wouldn’t they perform like a forcing cone to direct the energy to the base of the bullet?

 

[HTSmith]

Y'all are making my head hurt

 

[Alex Wheeler]

Soot does not bother me. But tight necks and tight end clearance does. Been down that road. I just do what the target likes and dont worry about the rest.

 

[Alex Wheeler]

Apparently the question has been answered to some and I missed it...the bullet moves before the case opens. Or so it would seem. Maybe it's opened for long, heavy bullets but not yet for the short and stubbies, or with a long freebore.

IDK, so I'm asking and would like a better understanding too. If it's open, neck tension, by whatever term, should not affect tune. If not, then yes, especially if jumping. I would conclude from that single statement, jamming is a good thing. Again, I'm asking. I think we must first conclude if the neck is open or not..or somewhere in between, before we can go forward with this subject but that's just my 2 cents and worth less,, probably.

Probably some of both. But the neck definatelly opens before the bullet is completely out of it.

 

[AlNyhus]

You're talking about an extreme, with very small length adjustment available. Right?
A portion of it's tension is coming from the shoulder, and a bushing there is merely extending the shoulder.

Actually not. And the base of the flat based bullet is fully .050 below the neck/shoulder junction. In essence, all that contacts the bullet is an .085 long 'ring' (the neck).

Once I got the first one up and running and did the tuning on it, much of what we accepted as 'fact' about case necks had to be looked at in a different light. Hoop strength, for example, proved to be a much bigger factor than we'd imagined. And the length of the 'hoop'...when talking about copper and brass interference...is not much of a player. -Al

 

[Dos XX]

I believe a bullet seating press that reads pressure measures two different things, constriction and coefficient of friction / surface condition of the inside of your neck. The trick is to know which one or how much of each it is measuring. If the surface condition of the inside of your neck is always the same, any variance on the readout should be constriction.

Take a case with your normal prep and seat a bullet. See what the read out says. Take another case and run a nylon brush in and out once and seat a bullet. See what the read out says. Try the same thing running the nylon brush in and out two time, three times, etc. and see what you get. Run a brush in and out by hand on one and with a power drill on another. If running light tension, it is possible to brush the ID of the neck enough to almost eliminate or even completely eliminate the pressure spike normally shown on a press with a readout.

Annealing adds texture to the inside of the necks. Tumbling with pins probably does, too. If you anneal, but not every firing, the surface finish could be different in your neck. Same with tumbling every time or not.

 

[PatMiles]

Tag

 

[AlNyhus]

At one time I was convinced that internal neck finish made a difference. What I found was that assuming a truly round inner neck with no taper...not as easy as it sounds...carbon in the neck was a bigger factor. -Al

 

[AlNyhus]

Al With regard to a bigger factor, do you find the carbon residue, as a negative or a positive factor?
Just trying to understand.. thx

IME, the carbon residue is a major positive. -Al

 

[mikecr]

But what about the shoulders? Wouldn’t they perform like a forcing cone to direct the energy to the base of the bullet?

Before the bullet is released, there is no flow. And pressure itself is not affected by form/shape.

Hoop strength, for example, proved to be a much bigger factor than we'd imagined. And the length of the 'hoop'...when talking about copper and brass interference...is not much of a player. -Al

It validates that you don't need a lot of sized area to hold bullets. That doesn't mean that grip beyond .085" doesn't count anymore. The entire length of necks, and even shoulder angles, contribute to grip.
Seating force measurements show this.
AMP press measure:

I use an instrumented mandrel die for my pre-seating operation.
Let's say my neck sizing length is 1/8".
I see mandrel seating force go up (like this AMP example) and level off as the mandrel bearing passes my sized portion of neck. I get an incredibly consistent force reading across that plateau.
If I extend neck sizing length to 1/4", I will still get a nice plateau, but higher. There is more grip on the bearing, and MV will go up with it.

 

[mikecr]

Yeah if firing did not leave a carbon film in necks, someone would be selling it to us.
It's a perfect neck lube.

 

[K22]

It depends on your total setup and what you are using the rifle for. I would never bother with turning necks for a field rifle. This gets back to the question of your total setup. Do you have all of the big stuff taken care of? Back in the day, for varmint calibers like my .222 and .223, after bedding and floating, and some trigger adjustment, loading with typical one piece dies, I reliably got under half MOA accuracy, well under. The question as to whether to turn depends on a lot of things, and if you do not given details about your total setup, it is impossible to give good advice.

This has been my 50+ years of precision varmint hunting / field shooting experience also, a lot of it with factory rifles but with tuned reloads.

I learned the hard way to address the bedding issue of the rifle first before attempting to test anything. Otherwise, you will be peeing in the wind. I found that once the rifle is properly bedded and the barrel free floated, this gave me the most consistent accuracy. It was not difficult to develop 1/2 moa loads in quality factory rifles using conventional FL dies properly and adhering to careful reloading practices to produce consistent reloads. My axiom in reloading, taught to me many years ago by my RIP mentor, is quality, not quantity.

I am a strong advocate of testing assertions for yourself using your rifle, your goals, and your skill level. My measuring standard is the shots on target, not theories or pronouncements by experts. If you don't test for yourself, you could end up chasing your tail, running around in circles. This applies for other aspects of the shooting sports also, for example cleaning.

 

[Coyotefurharvester]

You all are over looking a base factor, mass of the bullet. The bullet cannot move until the gas pressure is greater than mass of the bullet, no matter lube-lack of lube or "neck tension" or brass elasticity.

 

[wkdickinson]

I've read every post in this thread - lots of great info here. But I just can't get my arms around the "neck expanding to release bullet" concept. Logic tells me that when the powder ignites and starts developing pressure, that pressure ultimately overcomes the friction (neck tension and actual friction) in the bullet/neck, joint and the bullet moves. If that were not the case, then "squib" loads would not result in a bullet stuck in the barrel - I doubt a primer has enough energy to expand the neck of a cartridge sifiecently to release the bullet.

 

[Rank Amateur]

Logic tells me that when the powder ignites and starts developing pressure, that pressure ultimately overcomes the friction (neck tension and actual friction) in the bullet/neck, joint and the bullet moves.

I'm also trying to follow the arguments in this thread, and am leaning toward the expansion-release side. My reasoning lies in dimension of the neck after firing. If this were only the bullet moving forward when pressure behind it's base exceeds the friction holding it back, then wouldn't the post-firing inside neck size be the diameter of the bullet's pressure ring? Once the bullet moves out of the neck, gas (pressure) moves out into the chamber neck/throat/etc of the barrel. During this process, it also moves around the outside of the case neck (leading to the sooting discussed in previous posts). As the pressure is not directional - it's force is exerted on all surfaces exposed to it, then the distortion of the neck brass should stop, and the neck brass wouldn't be expanded to the inside dimension of the chamber, right? As the neck brass gets expanded well beyond the diameter of the bullet, that suggests that the expansion is taking place during the period when the pressure inside the neck far exceeds the pressure outside it. That suggests to me a "peeling" action (my highly inaccurate mental picture) where the expansion of the neck starts behind the bullet base and moves forward as pressure/gas works into the expanding space between the bullet sides and the case neck.

I believe that these events are rarely purely black and white, I would presume that at some point, the pressure behind the bullet exceeds the force retaining the bullet in the case and the bullet starts moving. I can see that starting to happen before the "wave" of neck expansion actually reaches the case mouth, so perhaps this is not 100% neck expansion around a stationary bullet that subsequently starts to move forward. Once the pressure/gas moves out of the case mouth and enters the forward extremes of the chamber, the pressure outside of the neck brass should prevent further expansion to the dimensions of the chamber, right?

As to the squibb issue, has anyone measured the neck of a fired case after a squibb?

 

[VA_XTC_Shooter]

Before the bullet is released, there is no flow. And pressure itself is not affected by form/shape.

It validates that you don't need a lot of sized area to hold bullets. That doesn't mean that grip beyond .085" doesn't count anymore. The entire length of necks, and even shoulder angles, contribute to grip.
Seating force measurements show this.
AMP press measure:
View attachment 1492944
I use an instrumented mandrel die for my pre-seating operation.
Let's say my neck sizing length is 1/8".
I see mandrel seating force go up (like this AMP example) and level off as the mandrel bearing passes my sized portion of neck. I get an incredibly consistent force reading across that plateau.
If I extend neck sizing length to 1/4", I will still get a nice plateau, but higher. There is more grip on the bearing, and MV will go up with it.

Mike, looking at how the AMP reports seating force, it still relies a lot on friction. If friction is not the big player in neck tension, rather case neck ductility and hoop stress are, why rely on the AMP? Obviously, neck ductility/hoop stress is accounted for somewhat in the AMP seater, but so is friction. So if we believe the case expands before bullet movement and friction is not a player, wouldn't comparing the case neck diameters of fired/resized for consistency be a better way to isolate the case neck ductility/hoop stress? It would take friction out of the equation.

 

[Webster]

pressure, expands neck to chamber walls, then bullet moves. Some gases move around bullet between case neck forward until bullet moves forward and seals against the forcing cone.

Why would the bullet seal against the forcing cone, it's larger in diameter the the bullet.

 

[VA_XTC_Shooter]

Why would the bullet seal against the forcing cone, it's larger in diameter the the bullet.

No it's not, the bullet is squeezed here into the lands and grooves and seals.this is where the engraving pressure occurs.

 

[Straightshooter1]

Probably some of both. But the neck definatelly opens before the bullet is completely out of it.

Agree that the neck opens before the bullet is completely out. My point in my post was that the bullet "begins" to move first, however minute, before the neck begins to expand . . . if the force it takes to expand the neck is greater than the force it takes to move the bullet. Unless there's some other force involved, like a crimp, the bullet will always start to move, a very small amount, before the neck starts to expand. . . or unless something like 30-60 lbs. of pressure can actually expand the neck first . . . which I doubt, unless something like the neck being fully annealed where it doesn't take much pressure to do so. I can think of an easy physics experiment, if one has the tools, to actually see this by pressurizing the cartridge from the base and measuring the neck and bullet's movement as the pressure increases. Certainly, if the bullet is touching or jammed in the lands, that's a whole different scenario.

 

[46and2]

We need a ultra high speed camera, a clear polymer chamber, colored powder inside of a case with a bullet seated, pressurized with compressed air.

The sine wave we see on the case neck is a sign.

I miss the Mythbusters!!!